[Reader-list] 7th posting

[sayandeb mukherjee]

here i lay the 7th posting. although it needs a proper
proofing for a verification of certain formulae
mentioned here...



7TH POSTING

In the previous posting we stated that the sound
generated ‘in’ a corridor which has two moderately
reflecting walls and relatively less an absorbent
floor and ceiling will produce flutter echoes. So for
a person (who himself is the acoustic centre) walking
with sharp and heavy boots or a lady walking down the
corridor with high heels will project deterministic
flutter echoes that can be perceivable if the ambient
noise level is relatively low. 
Flutter echoes is generally considered as an acoustic
defect as it affects speech intelligibility and tonal
coloration of music when acoustic treatment of
theatres, orchestra halls or recording studios are
taken into task. For example, if the time gap between
periodic reflections is 20 milliseconds, the space
will add a sound of 50Hz frequency [one reflection
every 20 ms means that 50 reflections will be received
in 1 s – a frequency of 50Hz.] to any sound produced
in this space thereby rendering a tonal aberration to
the direct sound or the source which could be a
acoustic musical instrument or voice. But here in a
space like corridor where the functional attributes or
the operational structure is completely different from
recording or performance spaces, it will not be
considered as an acoustic anomaly of the space. The
subjective perception of this phenomenon could result
in a liking or a disliking of the space. The
appreciations could be circumstantial as well and will
immensely depend on the temporality of the space. For
the late hours of the night, when the ambient noise
level has relatively diminished to a considerable
extent, other associative sounds of the corridor have
ceased to exist and the walker is alone with his
footsteps in the corridor, the fluttering sounds more
magnified than the normally perceived level. The
enhancement could be psychoacoustical for in the sound
field, the listener has no other audible object of
attraction/attention and the space appears to be
completely desolated for the time being. For this
lifelessness in the space created by a partial
acoustic vacuum and for the flutter echoes that sound
like sound shadows (the flutter echoes appear to the
listener as unseen shadows following his footsteps or
walking along with him), the walls appear to respond
to the user through this fluttering sound. It’s as if
the walls achieve to be a solitary living entity
thereby interacting with their master by mirroring his
activities. If the acoustic behavior of the walls is
uniform throughout the length of the passage then the
fluttering continues to exist unless the user changes
his volume or pattern of activity. Depending on the
psychic condition and habituation of the user, the
listener finds the phenomenon to be very annoying or
pleasant or unresponsive. 
Now when the acoustic origin depletes from the
observer, that is when somebody else is generating the
sound and he/she is departing from him there occurs a
continuous redemption of the direct sound level
reaching the ears of the observer. After certain
distance, the source will fade faster than its flutter
echoes thereby giving an unintelligible sound of those
footsteps. As the source (the detached acoustic
origin) reaches at a considerable distance, there
introduces an initial time delay gap – ITDG (see
figure) after which all the early reflections (the
foremost/first few reflected images from the walls and
other surfaces of the corridor discretely reaching the
ears of the observer) are audible. Then, immediately
after that, certain late reflections which are weaker
in volume/level and comparatively less discrete occur
in the space. After this, there commences
exponentially growing and smoothly decaying homogenous
sound field of uniform density. The phenomenon is
called reverberation. 
[Sound intensity is inversely proportional to square
the distance between the source and the listener. When
the distance is doubled the sound pressure level falls
by 6dB approximately which can be considered as a
substantial decline]

[Acoustic centre is the point in space from which the
spherical wavefront appeared to originate]


[Reverberation time:  the reverberation time of a room
is the time, in seconds, that would be required from
the mean squared sound pressure level therein,
originally in a steady state to decrease 60dB after
the source is stopped.  This time is symbolized as
RT60 gives a qualitative measurement during acoustic
treatment of recording spaces, auditoriums or
theatres]
[Reverberation: the phenomenon 

During this time phase, no reflection can be
discernible in a discrete fashion thereby resulting in
a smooth decay of the sound field to an inaudible
level. When the source reaches a distance called
critical distance, the direct sound level and the
reverberant sound level becomes equal resulting in a
total sound level LT that is 3dB higher than the
direct or reverberant sound level. 
If it is translated in terms of perception, one hears
an enhancement of the sound mixed with the wet sound
contributed by the reverberant sound at some point in
the space. As the source reaches the far end of the
corridor, the listener gets engulfed in a reverberant
field wherefrom the fluttering (as stated earlier) is
almost inaudible for: 
>1.It is masked by the dominant long tail of the
reverberation.
>2.The reflected images causing the fluttering are not
far reaching and die nearby to the source. The
reflections get absorbed by surfaces and by air before
it reaches our ears with sufficient energy. 
>3. Certain frequencies inherent in the sound source
get enhanced because of the normal modes of vibration
(discussed below) travel throughout the length of the
corridor. 
The pre-dominance of the long dense reverberation
gives a groovy impression of the space. And longer the
length of the corridor will dispense with a
corresponding long decay time of the reverberation. 

[Corridor space will have a long R.T. because of the
huge length like a columnar structure] 

In musical performances, reverberations added to
certain instruments (that have less staccato in its
playing style) provide a fullness of tone. It enriches
the tonal quality as the reverberation with an optimum
decay rate sustains the notes and fills the space
between them, causing a blending of successive notes.
Also for longer reverberation time in the lower
frequencies relative to the midband gives a pleasant
feeling of the source which is called warmth.
Interestingly, in a double loaded corridor space, when
two persons in proximity are interacting to each
other, and they reach distances from the distant
listener then a. the nuances of the discrete syllables
of their speech get muddled (like successive notes in
a musical instrument), b. the discernible harsh
reflections get absorbed by air or the walls before
reaching the listener, c. the high and hi-mid
frequencies are more absorbed than the low and
low-mids, d. at such distance, a homogenous
reverberant field is generated. 
So what he/she hears is a muffled, acoustically added
warmth, reinforced, smooth (as no reflection could be
discretely discernible), fullness of the voices (due
to long reverberation) that simulates a feeling that
somebody is murmuring with a mellowed voice. In this
case or for these sounds (e.g. Crowd, some people
chatting) it may, psychoacoustically, be very soothing
for the listener. Unlike, if it is an impulsive sound
– closing of a collapsible elevator gate or some heavy
thing falling on the floor, closing or banging of a
heavy door, shouting of a person generates a dense
long term reverberation engulfing the listener for a
certain period of time. This acoustically establishes
the depth of the corridor. If the listener is not
habituated, the phenomenon could be very annoying for
the listener and sometimes to such an extent that he
would make a retort as soon as possible. The
audio-attribute that projects the huge length of the
corridor is the terminal reverberance that emerges
after running reverberance terminates, i.e, some
moments after late reflections from surfaces (see
figure) die. It is this terminal reverberance along
with the imprint of TIME (the traveler or the stalker
will have his own subjective perception of time) that
the space appears as an instigator of exploration or a
repulsive, haunting sojourn. 
Along with all these phenomenon as discussed above,
there would be another acoustic phenomenon that
inevitably superposes with the direct sound thereby
further altering its tonal structure – the resonant
frequencies of the particular space. For being a
rectangular enclosure with an almost pipe-like
structure the space will have its corresponding
multiple resonant frequencies. Since, there are six
surfaces we should consider oblique modes of
vibration. But for having a tubular look, we presume
that axial modes will be predominant. The physical
significance of the phenomenon is that the room gives
preferential treatment to a resonant frequency or
modal frequency (popularly called) by amplifying the
sound at that frequency. So for a sound with more or
less appreciable spectral content and width, when
produced in a corridor space, as an effect of the
preferential treatment to certain modal frequencies
(contained in the source, of course) by virtue of
resonances, there occurs tonal coloration/aberration
of the source. The coloration is, once again, variable
with the distance of the listener from the source. So
the listener standing at different points in the space
perceives different colorations of the same source.
Along with the length of the corridor, there also
happens a variation in sound pressure level because of
this phenomenon. So the listener hears a. tonal
coloration of the source, b. different loudness at
different points in the space. For the existence of
modal frequencies, the reverberation of the sound also
changes its pitch, in some cases. The reason is that
after the source is cut off, the room will have no
memory of the frequencies in the source that was being
generated. The sound energy, therefore, resolves into
the closest modal frequencies resulting in a frequency
shift and hence a tonal coloration. 

Stating these entire phenomenons (acoustic) that a
acoustic wave confronts in a narrow passage like
corridor could be very convenient, yet the behavior of
sound while making a long journey from one end to the
distant other is complicated, overlapping and
undeterminable for many reasons like – 1.variations in
absorption coefficients of different surfaces,
interfaces (like doors, windows), 2.different
dimensions of the corridors or differences in its
spatial configuration, e.g., it can have many
side-links, to many other outlets, or it could end in
a T or L or a dead end, 3.number of windows, doors
giving openings to rooms of indefinite dimensions. So,
it would be an unjust to resolve all these acoustic
qualities that particularize a corridor space, to a
conclusive, authoritative treatise that would state -
these will occur and others not for any corridor space
on earth. Also it is impractical to consider all case
which will be enumerable where a corridor space will
have certain discriminating, peculiar, individualistic
soundscape that differentiates one from the other
immensely. 
Here, the attempts have been made to envelop the most
general, common and striking acoustic phenomenons in
the space.
As an extension of the case of double loaded corridors
we find that TUNNELS are also worth mentioning in this
regard. We can say that unless the waveform is very
complex, a tunnel behaves like a narrow pipe: the
similitude could also be drawn with a flute
.contd.

It is inconclusive in this regard, yet is surely
mentionable here that the unusual exaggeration of
acoustic elements and their different processes of
enhancement and termination in the spatio- temporal
domain may impart with certain psychedelic
manifestations. We now try to portray what is
happening in the psychological domain of an individual
as he makes his journey through this kind of space by
studying the following cases and sub-cases...contd 

sayandeb




SAYANDEB MUKHERJEE
  FT#308, SUBBARAJU TOWERS,
  ROAD NO.4, VIJAYAPURI COLONY,
  KOTHAPET, HYDERABAD
  PIN: 500 035
  PH#9849383863



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